umiddelb

More or less. Use a tagged VLAN for the WAN port (e.g. 7) and an untagged VLAN for the LAN port. Define an untagged switch-port with the same VLAN-ID (7) and connect your WAN-uplink to this port. Some ISPs require a tagged switch-port in order to communicate with their modems, e.g. Deutsche Telekom requires VLAN-ID 7. ProxmoxVE offers a nice UI for virtual networking.

I'd start with a lxc container image, e.g. from here: https://images.linuxcontainers.org/ , instead of a VM.

There are some distributions like pfsense or OpenWRT for that particular use case. You may consider to run a virtualized instance of them on top of proxmox. As well as you might want to run a NAS instance in a different container instead of mixing them up in single installation. Armbian has added proxmox support recently (https://docs.armbian.com/software/proxmox-ve/) A usual setup may contain: OpenWRT or Pfsense AdguardHome or PiHole WiFi-controller depending on your wifi hardware, e.g. unifi OwnCloud, NextCloud, OpenCloud, OpenMediaVault, ... DoveCot If your SBC has only one nic, you can still build a router with the help of a VLAN capable switch.

homeassistant ?

I'm running Proxmox VE on small x86-64 hardware with multiple nics and then various LXC containers / VMs. One for OpenWRT, Unifi, Adguard Home, NextCloud, HomeAssistant, Freifunk-Offloader, ...

What UART parameters have you set, especially flow control?

There is an implicit timeout built into uboot which triggers this error messages. I suppose that the newer firmware will circumvent this timeout.

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